Uruguayan President Jose Mujica has said the start of legal cannabis sales will be delayed until next year due to "practical difficulties".
Sales in state-owned pharmacies were due to begin as early as November.
But in an interview with the AFP news agency, Mr Mujica said government-produced marijuana could end up in the illegal market if implementation of the law was rushed.
The United Nations has criticised the law, saying it posed a health danger.
"If we want to get this right we are going to have to do it slowly," Mr Mujica told AFP.
"We are not just going to say, 'hands off and let the market take care of it,' because if the market is in charge, it is going to seek to sell the greatest possible amount," he added.
Uruguay has become the first country to make it legal to grow, sell and consume marijuana.
The legislation was approved by the Uruguayan Congress in December and came into force in May.
Regulation was expected by the end of this year.
Licensed pharmacies will sell the drug for less than $1 (£0.65) a gramme, with consumers allowed 40g (1.4oz) a month.
The bill specifies that each household may grow up to six cannabis plants, and that marijuana may be consumed in the same places as tobacco.
Uruguay's government hopes the bill will help tackle drug cartels, but critics say it will expose more people to health risks and other drugs.
Mr Mujica, a former member of the Tupamaros left-wing rebel group in the 1960s and 1970s, has introduced a number of liberal measures since being elected in 2009.
He will leave office at the end of his term next March.